If he wins, President Obama may strengthen his hand at home. If he loses, the Syrian people will bear the consequences.
Congress vote will send the wrong message to Assad
In the end, the constitutional lawyer won the argument with the commander in chief: Barack Obama decided the risks of striking Syria without the explicit backing of Congress were greater than the risk that America's lawmakers would deal him a humiliating setback by voting no.
In his short Rose Garden speech on Saturday, the president who ran as the candidate of peace threatened to make war against Bashar Al Assad, but meanwhile demanded that the people's representatives stand up and be counted - Mr Obama will have none of the endless sniping that followed his Libya operation, before which Congress was merely consulted.
It is, to say the least, an enormous gamble - perhaps the most consequential of Mr Obama's presidency. The White House is projecting confidence, but there's a very real chance that it will lose this vote. And make no mistake: A loss would prove devastating to Mr Obama's prestige at home and abroad, scuppering the remainder of his domestic agenda and deepening the doubts gnawing at America's allies.
In the House, where Mr Obama faces entrenched opposition to nearly anything he does, looming budget fights are further poisoning an already toxic atmosphere. Earlier this summer, the Republican-dominated House stunned Washington when it failed to pass a farm bill, usually a routine matter. Immigration reform, despite the strong backing of both party establishments, seems doomed to a slow, grinding death. An amendment severely restricting the once-invulnerable National Security Agency surprised everyone in July when it nearly passed with a strange-bedfellows coalition of libertarians and the left.
The House debate over Syria will be waged along the same battle lines - and the vote will be just as close. Already, Republican aides are signalling that Democratic votes will be needed to secure a yes vote. Minority leader Nancy Pelosi has voiced her strong support of the president, but many Democrats, still furious over the fiasco in Iraq and very much aware of the public's distaste for deeper involvement in the Middle East's bloody conflicts, will ignore her entreaties. How many she needs is anyone's guess.
The Senate seems like an easier sell, with 53 generally reliable Democratic senators and a hawkish bloc of Republicans led by John McCain and Lindsey Graham. But even there, authorisation is no slam dunk: A single senator, most likely Kentucky's Rand Paul, could launch a filibuster, forcing Majority Leader Harry Reid to round up 60 votes. And Mr McCain and Mr Graham have warned that they will vote against a strike plan that is not tied to a comprehensive strategy for Mr Al Assad's ouster, raising the prospect that the hawks could be the tougher political problem for a president intent on keeping his intervention, in the words of one embittered official, "just muscular enough not to get mocked".
What seems inevitable - at a minimum - is that the White House's expansively drafted authorisation will emerge in much narrower form. Sunday's Capitol Hill briefing, the White House ran into a buzzsaw of scepticism, with wayward lawmakers loudly declaring themselves unpersuaded by the administration's case for action, while advocates remained largely silent. Senator Patrick Leahy, the influential head of the Senate Judiciary Committee, said he would amend the White House draft, and others will doubtless offer changes of their own.
The coming week, or weeks, of high-stakes drama in Washington will make for gripping television and perhaps a healthier democracy - but hardly a potent message to the Syrian regime, which has gradually escalated its brutal counterinsurgency campaign as the international community has largely tut-tutted from the sidelines.
Two years ago, how many would have guessed that, after 110,000 deaths and the gassing of hundreds of Syrian civilians, Mr Al Assad would still be in power - and his forced ouster wouldn't even be on the table? Certainly not Mr Obama, who clearly thought it would all be over by now.
What's particularly baffling is that the White House seemed woefully unprepared for this moment, despite months of repeated warnings of the terrible consequences Mr Al Assad would face should he cross the president's red line. As Fred Hof, a recently retired State Department expert on Syria, wrote in a scathing blog post: "The events of the past 10 days suggest that there was no administration forethought to the possibility of a major chemical incident in Syria; there was no plan in place to respond to a major chemical attack by a regime that had already demonstrated its deep and abiding contempt for the president and his red lines."
Even more puzzling, the administration has promised to punish Mr Al Assad even if Capitol Hill demurs. "We don't contemplate that the Congress is going to vote no," John Kerry, the secretary of state, said on Sunday, adding that the president retained the power to order strikes "no matter what Congress does".
It is no exaggeration to say that the remainder of Mr Obama's presidency hinges on this vote.
If he wins, the president may have strengthened his hand at home, even if his indecision has done damage to his standing abroad. But if he loses, it is the Syrian people who will bear the consequences.
Blake Hounshell is former managing editor of Foreign Policy
On Twitter: @blakehounshell